(1) Field of the invention:
This invention relates to a III nitride film, particularly to an Al-including III nitride film which is suitable for a light-emitting diode or a high speed IC chip.
(2) Related Art Statement:
Al-including III nitride films are used as semiconductor films for light-emitting diodes, and recently get attention as semiconductor films for high speed IC chips of cellular phones.
Conventionally, such an Al-including film is fabricated by a MOCVD method in which trimethyl aluminum (TMA) and triethyl aluminum (TEA) are employed as an Al supply source, and ammonia (NH3) is employed as a nitrogen supply source.
In this case, a substrate on which the III nitride film is formed is set on a susceptor installed in a reactor and is heated to around 1000xc2x0 C. by a heater provided in or out of the reactor. Then, the Al supply source, the nitrogen supply source and another supply source including another additive element, with given carrier gases, are introduced into the reactor and supplied onto the substrate.
Just then, the supply sources thermochemically react on the substrate, and the thus decomposed constituent elements chemically react to deposit a desired Al-including III nitride film on the substrate.
However, when the Al-including III nitride film is formed directly on a single crystal substrate made of C-faced sapphire single crystal, C-faced SiC single crystal or the like or an epitaxial substrate constructed of the same single crystal and an underfilm made of III nitride, some cracks may be created in the film, irrespective of the composition, depending on the film-forming condition and the film-forming apparatus condition such as the reactor configuration and size.
Therefore, the yield ratio of elements constructed of such single crystal substrates or epitaxial substrates and the Al-including III nitride films formed on the substrates is degraded.
It is an object of the present invention to provide an Al-including III nitride film without cracks when it is formed directly on a single crystal substrate or an epitaxial substrate.
In order to achieve the above object, this invention relates to a III nitride film including at least Al element and having hexagonal crystal system which is directly grown on a crystalline substrate along the C-axis, the lattice constant xe2x80x9ccxe2x80x9d of the main axis of the III nitride film and the lattice constant xe2x80x9caxe2x80x9d of the crystal face perpendicular to the main surface of the substrate satisfying the relation of xe2x80x9cc greater than 2.636a-3.232xe2x80x9d. Here, xe2x80x9calong the C-axisxe2x80x9d includes subtle tilt of the C-axis originated form deviation of growth conditions, subtle off-angle value of crystalline substrate and so forth.
In a preferred embodiment of the present invention, the lattice constant xe2x80x9ccxe2x80x9d and the lattice constant xe2x80x9caxe2x80x9d satisfy the relation of xe2x80x9cc greater than 2.636a-3.221xe2x80x9d. In this case, even though the III nitride film is thermally shocked, no cracks are created in the film.
The inventors intensely studied to obtain an Al-including III nitride film without cracks when the III nitride film is formed directly on a crystalline substrate. Since the cracks in the III nitride film occurred, irrespective of the composition, depending on the film-forming condition and the film-forming apparatus condition, the inventors intensely investigated the physical properties of the film and substrate which influence the creation of cracks.
As a result, the inventors discovered that the crack creation in the III nitride film directly formed on the crystalline substrate relates to the sizes of lattice constants of the crystal lattice of the III nitride film. That is, in the hexagonal crystal lattice of the III nitride film, if the lattice constant xe2x80x9ccxe2x80x9d of the main axis and the lattice constant xe2x80x9caxe2x80x9d of the crystal face perpendicular to the main axis satisfy the above relation, the crack creation can be inhibited even though the III nitride film is formed directly on the crystalline substrate. This invention results from the above enormous research and development.
FIG. 1 is a graph showing the correlation between the lattice constants and the crack creation in a III nitride film having a composition of AlxGayInzN (x+y+z=1, x greater than 0). As is apparent from FIG. 1, if the relation of xe2x80x9cc greater than 2.636a-3.232xe2x80x9d is satisfied, cracks are not created in the film. On the other hand, if the relation is not satisfied, some cracks are created in the film.
Herein, in FIG. 1, ∘ plots designate that cracks are not created and x plots designate that cracks are created.